The NE555 monostable circuit

Figure 1: Timing diagram of the NE555 monostableIn issue 3 the first of the three basic circuits that can be built with the NE555 IC, the astable, was explained. In this issue, we look at the monostable circuit which can be used as part of a ‘push-to-operate’ system for a model at exhibitions.

Click here to read this datasheet in Adobe Acrobat format 555 Timer Datasheet
141Kb

When the monostable circuit is powered up, the NE555 Output (pin 3) will be 0V. It will remain at 0V indefinitely if the user does nothing. When a pulse is applied to the trigger input (pin 2), the Output goes high to Vs for a period of time called the ‘time period’. After this time the Output returns to the low, 0V, state. All of this is shown the timing diagram of figure 1.

Figure 2: Circuit diagram of the NE555 monostableCalculating the time period

From the circuit diagram of figure 2, you can see that there is a resistor R and a capacitor C which determine the time period.
The equation…

Equation T = 1.1RC

…relates the time period T in seconds to the capacitor value C in Farads and resistor value R in Ohms.

You can use the steps below to calculate the value of R. An example is shown in {curly brackets}:

  1. Decide on the time period T which you require. This can be anything from milliseconds to several minutes, although you must express it in seconds. {I choose T=10s}

  2. Next, guess a value for the capacitor C in farads. For starters, try 100mF. {I choose C=100mF}

  3. Put the values of T and C into the equation below and calculate resistor R

    Equation R = T / 1.1C

If the value you get for R is not between 1kW and 1MW, select a different value for C and do the calculation again.

To reset the monostable circuit after it has been triggered, and before the time period has elapsed, you can momentarily connect the Reset pin to 0V.

An example stripboard layout for the circuit is shown in figure 12.

Need some help with these equations? Why not get the monostable wizard to do the work for you!
Only works with Microsoft Internet Explorer version 4.0 and above.

See also: The NE555 astable circuit for an explanation of the NE555 timer IC

Contrast with: The NE555 astable circuit , The NE555 bistable circuit

The Electronics in Meccano Circuits Shop Buy 555 Monostable Modules from the Electronics in Meccano Circuits Shop

Click here for details

 

Figure 12: Stripboard layout for the NE555 monostable circuit

 

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Some NE555 tricks

Here are a few extra techniques you can use with your NE555 circuits to make them easier to use and to add extra features…

Figure 3: Sinking and sourcingSinking & Sourcing

In the two circuits we have discussed so far, the component to be controlled, such as an LED, lamp or relay, has always been connected between pin 3 of the NE555 (its Output) and 0V. This is called ’sourcing’ and means that the component is switched on when the NE555 Output goes high.

If you instead connect the component between pin 3 and +Vs, it will be switched on when the NE555 is off. This technique is called ‘sinking’. It is possible to use both sinking and sourcing in the same circuit — try the astable circuit from issue 3 with two LEDs connected to the output as shown in figure 3. The LEDs will flash on and off alternately. Remember to put in the correct LED resistors or, as you can see, there will be a direct current path from +Vs to 0V through the two LEDs — and they won’t like that very much!

Figure 4: Variable resistorsUsing a variable resistor

To make an astable or monostable circuit with a variable frequency or time period, rather than fixed, you can replace one of the timing resistors (R, R1, or R2) with a variable resistor (also called a ‘potentiometer’ or ‘pot’.)

There are two types of variable resistor — linear and logarithmic. For this application the linear type (usually marked with ‘LIN’) is the most appropriate. The value marked on the variable resistor is its maximum resistance when the shaft is rotated fully, so you can still use the various equations to determine the lowest frequency or longest time period that the astable or monostable circuit will give. Because resistances in NE555 circuits should never be below 1kW, a 1kW resistor should be placed in series with the variable resistor.

In the astable circuit, you will find that replacing R1 with the variable resistor will vary the ‘on’ time of the NE555, and replacing R with the variable resistor will vary both the ‘on’ and ‘off’ times of the NE555.

Standard variable resistors will have three connections — the middle one (connected to the ‘wiper’) should be used along with any of the others. Variable resistors can be panel mounted (usually a 10.5mm hole) and have a plastic shaft which can be cut to the required length to fit a knob.

Other types of variable resistor

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What's m and k?

In electronics and science in general letter and symbol prefixes to units of measure are used to save having to write very large or very small numbers in full.

For example, the value 1,000,000W can be abbreviated to 1MW, the ‘M’ meaning ‘multiply by 1 million‘. In the same way, the value 0.000001F can be written as 1mF, the ‘m’ meaning ‘divide by 1 million’.

In all the equations we have been discussing in the articles, the values you use must have their prefixes removed by multiplying/dividing as appropriate.

Below is a list of the prefixes you may come across, how you pronounce their names, and what you have to do to get the value without the prefix:

Prefix Name Meaning To get value
p pico (1 x 10-12) Divide by 1 trillion
n nano (1 x 10-9) Divide by 1 billion
m micro (1 x 10-6) Divide by 1 million
m milli (1 x 10-3) Divide by 1 thousand
k kilo (1 x 103) Multiply by 1 thousand
M mega (1 x 106) Multiply by 1 million

An Ohm’s Law example

Suppose you have a 3.3kW resistor with a current of 2mA passing through it and you want to find out what voltage is across it.
You can use Ohm’s Law...

Equation V = IR

...but only after you have removed the prefixes from the values...

Equations V = 2mA x 3.3k | V = 0.002A x 3300R | V = 6.6V

...and because every value was used in its basic unit, you can be sure that this answer is 6.6V.

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Building a power supply

Most of the circuits we have built in Electronics in Meccano need a smooth DC power supply. Some other circuits, such as those using logic IC’s, also need their supply to be regulated.

Although you may have a mains transformer that steps-down the 230V AC supply (the UK mains power supply voltage), this may not have built-in smoothing or regulation.

Figure 5 shows a block diagram of the parts of a power supply system which turns a 230V AC mains supply into a regulated 5V DC supply. Assuming that you already have a suitable mains power supply with a transformer, such as those used to control model trains (I wouldn’t recommend building your own because of the safety considerations), you will need to build the rectifier, smoothing and possibly the regulator blocks.

If the mains power supply that you start with does not have a cut-out or a fuse, add one of an appropriate rating as shown in the circuit diagram below.

Figure 5: Block diagram of a power supply system

What is AC and DC?

A representation of an AC (Alternating Current) supply is shown on the right. The voltage changes from positive to negative and back again over time.
In the case of the mains supply this will happen 50 times per second (50Hz).
Figure 6: An AC Waveform
Figure 7: A DC Waveform A DC (Direct Current) supply, shown left, stays at a fixed voltage all of the time (like the voltage from a battery).

Parts of the power supply system

Figure 8: RectificationA full-wave rectifier is made using four diodes wired up as shown in the left-hand part of figure 11. A diode lets current pass in only one direction through it, so the arrangement of the four diodes means the positive parts of the wave-form are routed to the +12V output and the negative parts are routed to the 0V output.

Unfortunately, the output is not steady DC as it should be because the input voltage to the rectifier is always changing and sometimes it is zero, resulting in a waveform like the one in figure 8.

Figure 9: SmoothingThe output has to be smoothed by placing a large value capacitor across the output of the rectifier. The capacitor charges up as the voltage rises and releases it’s energy slowly during the periods when the supply voltage falls. The result is almost DC, but there is still a small ‘ripple’ voltage. Most circuits can tolerate this, but some Integrated Circuits need the supply voltage to be rock-steady.

To do this, a regulator IC is needed. These can take a range of DC voltages and produce a steady DC voltage. We will use the 78xx series of regulators which are wired up as shown on the right of figure 11. Choose the correct regulator for the voltage you require; 7805 for +5V (needed for TTL logic ICs), 7812 for +12V or 7815 for +15V.

Figure 10: Pin-out diagram of the 78xx regulatorsThe 78xx regulators can supply a current of up to 1A, and there are of course other types of regulator that deliver different voltages and currents.

Click here to read this datasheet in Adobe Acrobat format 7805 Regulator Datasheet
72Kb

Note that this is a very simplified explanation of power supplies since the output voltage of each stage (rectifier and smoothing) will not be exactly the same as the input voltage to that stage. Always check the output voltage before you connect the power supply to a circuit.

 

More on this in the next issue.

To view figure 11 full size, please click on it.
Figure 11: Circuit diagram of the power supply system

Figure 13: A stripboard layout for the simple power supply

Continued in: More about power supplies which gives more details about smoothing and rectification.

The Electronics in Meccano Circuits Shop Buy 5V Regulated Power Supply Modules from the Electronics in Meccano Circuits Shop

Click here for details

 

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Title: Practical Matters -- Connectors

Wander SocketThere are many different types of connector that you can use in your models to allow parts of the model or circuit to be easily dismantled and assembled. A typical example would be in a crane, where the jib may be removable, or a control panel that can be separated from a model.

Single-way connectors

Wander PlugThe following types of connector allow only one wire to be connected. These are generally used in applications where only a few connections are needed (such as the two power supply connections to model), or for connections that are frequently moved around (some designs of control panel.)

Wander Plugs

Banana Plugs

In issue 5, Practical Matters will take a look at the different types of multi-way connectors such as DIN plugs, D plugs and PCB pin strips.

See also: Practical Matters: Connectors for multi-way connectors

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Title: Shopping ListThe following lists the electrical parts that are discussed in the articles. Prices and order codes given are taken from the current Maplin catalogue, which is the probably best source of electronic components for the hobbyist in the UK.

If you have access to a company account with Rapid Electronics or RS Electronics you may find these companies are cheaper.

The NE555 monostable circuit
 
Parts needed for the NE555 monostable circuit in Figure 2 to be constructed on stripboard
Maplin Order Code Price Page
1 x NE555 Timer IC QH66W 29p 292
1 x 8-pin IC Holder BL17T 14p 304
1 x 220mF Capacitor VH41U 22p 94
1 x 0.01mF Capacitor BX70M 15p 91
1 x 1kW Resistor M1K 7p 221
1 x Push button FH59P 59p 314
Stripboard (about 24 holes x 12 rows) JP47B £1.79 204

Also needed are suitable values for resistor R and capacitor C.

Some NE555 tricks
Maplin Order Code Price Page
Standard 100kW Potentiometer FW05F 89p 226
Preset 100kW Potentiometer UH06G 20p 224
1kW Resistor M1K 7p 221
Building a power supply
 
Parts needed for the 5V regulated PSU circuit in Figure 11 to be constructed on stripboard
Maplin Order Code Price Page
4 x 1N4001 Diode QL73Q 5p 246
2 x 0.1mF Capacitor BX76H 15p 91
1 x 1000mF Capacitor VH50E 44p 94
  7805 1A Regulator or: CH35Q 39p 270
  7812 1A Regulator or: CH36P 39p 270
  7815 1A Regulator CH37S 99p 270
20mm 1.6A Quick-blow fuse (pack of ten) GJ92A 80p 174
20mm Fuse holder with cover DA29G 24p 177
Practical Matters: Single-way connectors
Maplin Order Code Price Page
3.2mm Wander plug black HF50E 49p 142
3.2mm Wander plug green Discontinued
3.2mm Wander plug red HF53H 49p 142
3.2mm Wander Socket black HF56L 49p 142
3.2mm Wander Socket green HF58N 49p 142
3.2mm Wander Socket red HF59P 49p 142
4mm Banana Plug (stackable) black JM00A 74p 142
4mm Banana Plug (stackable) blue JM43W 74p 142
4mm Banana Plug (stackable) green JM44X 74p 142
4mm Banana Plug (stackable) red JM45Y 74p 142
4mm Banana Plug (stackable) yellow JM46A 74p 142
4mm Banana Socket (panel mounting) black HF69A 49p 142
4mm Banana Socket (panel mounting) blue HF70M 49p 142
4mm Banana Socket (panel mounting) green HF72P 49p 142
4mm Banana Socket (panel mounting) red HF73Q 49p 142
4mm Banana Socket (panel mounting) white HF74R 49p 142
4mm Banana Socket (panel mounting) yellow HF75S 49p 142
4mm Banana Socket (PCB mounting) Discontinued

Maplin charge £2.50 for delivery on orders under £30.00 inc. VAT.
Prices are taken from the September 2000 - August 2001 Maplin catalogue, and include VAT at 17.5%

Contact their order line on 0870 264 6000 or visit one of their shops.
Their customer service line is 0870 264 6002 and they have a website at www.maplin.co.uk where on-line ordering is available.

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Electronics in Meccano June 1999 -- Issue 4

Edited by Tim Surtell
E-mail: timsurtell@eleinmec.freeserve.co.uk


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